Medical ventilators have been used for a number of years to simulate a breathing cycle of patients unable to breathe on their own. In some instances ventilators provide temporary assist for defined periods, for example, when normal patient breathing is interrupted by a surgical procedure. In other instances, ventilator use can be much longer term. Particularly with long-term ventilator use, conventional patient circuits associated with the ventilator do not allow patients to produce an audible laryngeal voice. This is because typically the patient is coupled to the ventilator by a patient circuit including a cuffed tracheotomy tube inserted into the trachea of the wearer below the level of the vocal chords. The cuff on the tracheotomy tube is inflated, for example, with air, so that the cuff seals substantially fluid tight against the wall of the trachea. The effect of the cuff is to provide a closed mechanical respiratory system that completely bypasses the upper airway above the level of the tracheotomy tube, including the vocal chords. One result is the elimination of exhaled airflow up through the vocal chords.
To enable such patients to produce audible laryngeal voice, valved tracheotomy tubes have been developed. One example is a valved-fenestrated tracheotomy tube having an inner and outer cannulae described in Blom, U.S. Pat. No. 6,722,367, the contents of which are incorporated in their entirety herein. Such a valved tracheotomy tube can be used to divert expiratory air from a patient circuit associated with a ventilator up through the wearer's vocal chords, mouth and nose, permitting audible vocal chord vibrations for speech. The Blom valved tracheotomy tube has significantly improved the quality of life for many long-termed ventilated patients by enabling patient speech using their own vocal chords. However, one problem with the Blom device and any other system diverting expiratory air from a patient circuit is the volume of expiratory air in the patient circuit is greatly reduced if not eliminated. However, virtually all ventilators utilize flow transducers to continuously monitor the volume of expiratory air in the patient breathing circuit. If the transducer detects that the volume of expiratory air drops below a minimum volume threshold, an alarm is generated. The alarm is intended to warn an operator there is an inadequate supply of expiratory air in the system which could mean, among other things, a leak in the system. Thus, diverting expiratory air to promote patient speech can have the undesirable effect of generating an alarm signal indicating an inadequate volume of expiratory air. Due to the life critical nature of ventilators, the expiratory air volume detection system cannot simply be disabled.
The present invention is directed to overcoming one or more of the problems discussed above.